Thyrotropin Releasing Hormone (TRH) shows the obvious and long-durated stimulation to the neonatal thyroid tissue through the pituitary. The sequential changes of serum thyroglobulin (Tg) levels and light and ultrastructural change in the cells of the thyroid gland were studied after TRH injection into neonatal rats. TRH injection induced a rapid increase of the colloid droplets in the thyroid follicular cells followed by a slow increase in serum Tg levels. Ultrastructurally, although marked surface activities consisting of formation of pseudopods and colloid droplets were observed after TRH injection, luminal colloid did not appear to go across the intercellular space between the thyroid follicular cells.